The present invention is directed to a sheet product and to cathodic electrode products for non-aqueous battery formed from said sheet product.
Storage batteries have a configuration composed of at least one pair of electrodes of opposite polarity and, generally, a series of adjacent electrodes of alternating polarity. The current flow between electrodes is maintained by an electrolyte composition capable of carrying ions across electrode pairs. In addition to these active components, there must be an inert material separating the electrodes of opposite polarity. Separators have been used in many forms including grids, blocks, sheets and the like formed from nonconductive materials.
Non-aqueous batteries have certain distinct advantages over other types of storage batteries. They use, as anodes, light weight metals, normally composed of alkali metals or their alloys such as lithium, or lithium-aluminum alloys and the like which are at the far end of the electromotive series. These batteries have the potential for providing much higher gravimetric and volumetric energy densities (capacity per unit weight and volume, respectively) than other types of batteries due to the low atomic weight of the metal and high potential for forming a battery in conjunction with suitable positive electrodes far removed from the light weight metal electrode (the description herein will use batteries having lithium as the light weight metal anode although other light weight batteries having lithium as the light weight metals can be used) in the electromotive series. The battery can be formed in any conventional physical design, such cylindrical, rectangular or disc-shaped "button" cells, normally of a closed cell configuration.
The battery components of positive electrode, negative electrode and separator can be in the form of distinct alternating plates in a sandwich design or of a continuous spirally wound design as are well known. The anodic electrodes can be formed, for example, from a light metal, such as lithium or its alloys, on a support, such as a nickel coated screen. The electrolyte can be formed of a non-aqueous solvent, fused or solid electrolyte. Illustrative of known useful non-aqueous solvents include acetonitrile, tetrahydrofuran and its derivatives, propylene carbonate, various sulfones and mixtures of these solvents containing a light metal salt such as lithium salts, as for example lithium perchlorate, iodide or hexafluroarsenate and the like.
The cathodes are normally formed with one or more metal chalcogenide compound such as sulfides, selenides, selenides and tellurides of titanium, vanadium, hafnium, niobium, zirconium and tantalum. Typically the chalcogenide active materials are produced in particulate form and pelletized or chemically bound up to form a cathode structure configuration. One of the most common manners of binding the cathode active material involves the use of Teflon [poly(tetrafluoroethylene)] as the principal or sole binding agent. Teflon bonded cathodes have certain drawbacks which limit their ability to provide a highly effective cathodic electrode. For example, they are fabricated from aqueous slurries which both limits the type of chalcogenides that can be used (the most desired chalcogenide, TiS.sub.2, is unstable to water) and is a cause for concern when used in batteries having a light metal anode, such as lithium. Further, Teflon formed cathode products are rigid structures not suitable for formation into a variety of desired shapes and configurations. Patents illustrating this technology include U.S. Pat. Nos. 3,457,113; 3,407,096; 3,306,779; 3,184,339 and 3,536,537.
Other polymer binders have also been suggested to aid in forming chalcogenide cathodic electrodes. For example, EDPM (ethylene-propylene-diene terpolymer) and sulfonated ionomers have been used to form cathodic electrodes by slurry processes. Although the resultant electrodes exhibit greater elasticity and flexibility, they still have major defects of non-uniformity, poor control of pore size distribution and porosity and severe loss of activity after being subjected to only a few charge/discharge cycles as shown by the low figure of merit reported in U.S. Pat. No. 4,322,317 which illustrates this technology.
Finally, an important component of a battery is the separator. This component is normally in the form of a separate sheet material inserted between electrodes of opposite polarity to prevent their contacting one another. In batteries, such as presently described, the separator must be inert with respect to the other components, be capable of permitting electrolytic conduction through the separator and, in secondary batteries, it must be able to inhibit and prevent dendritic shorting. Because of the types of polymers used in forming the cathodic electrodes, especially inert Teflon, the separator has been a separate component. This causes additional effort in assembling the cell. Further, its effectiveness is impaired as the separator tends to shrink or migrate thus allowing exposed areas of electrodes. Finally, such individual separator membranes do not provide a means of effectively inhibiting dendrite formation and shorting therefrom. Normally, the edge portions of the electrodes remain exposed to permit dendrite shorting. To overcome this, it has been proposed that the light metal electrode be encapsulated by a separator type envelope. However, this has many complications due to the high reactivity of the light metal, especially lithium.
It is highly desired to form a sheet product suitable for use in forming a cathodic electrode product composed of a polymer-bonded cathodic electrode which is integrally bonded to and encompassed by a composition capable of functioning as an inert microporous separator membrane. It is further highly desired that the sheet product and the cathodic electrode product have good mechanical integrity, be flexible and capable of being formed into various configurations required for different battery designs. It is still further highly desired that cathodic electrode product permit high utilization of the active material and maintain the high activity after subjection to a multiplicity of charge/discharge cycles.